This invention relates to message store-and-forward communication systems and particularly to radio-frequency store-and-forward communication systems which operate typically over a predetermined, limited number of available communication channels. Features of the invention are found to have particular application in satellite communication systems operating in a "star topology" over a fixed number of channels.
The invention herein is described as an improvement over a well known prior art system which is known as the "Standard-C communication system", see THE STANDARD-C COMMUNICATION SYSTEM, N. Teller et al. International Maritime Satellite Organisation, London, England, International Conference on Satellite Systems for Mobile Communications and Navigation, 4th, London, England, Oct. 17-19, 1988, Proceedings (A89-36576 15-32). London, Institution of Electrical Engineers, pp. 43-46, (1988). A communication system such as the Standard-C system operates in a "star topology", hence between a "hub" and a substantial number of "mobile terminals". The Standard-C system utilizes schemes known as Time-Division Multiple-Access (TDMA) and Frequency-Division Multiple-Access (FDMA) to accommodate a large number of intermittent users (intermittently used mobile terminals) to share a limited-bandwidth, allocated, radio frequency band.
In FDMA, the allocated band is divided into a first number of narrow sub-bands, each sub-band constituting a time-continuous channel. The first number of channels are to be shared among a second number of users, where the second number, the number of users, is typically much larger than the first number of channels. An access protocol referred to as trunking is used to accommodate the relatively larger number of users.
In TDMA, an entire allocated band forms a wideband communication channel which is allocated to different users at different times. Data packets from a given user may be interspersed with those of another user during transmission over the communication channel.
Known communication systems, including the above-identified Standard-C Communication System, use a combination of FDMA and TDMA, where time-division multiplexing is used in the frequency-division multiplexed sub-bands. In the Standard-C system, forward (from the hub to the mobile terminals) data traffic is carried in a time-division multiplexed (TDM) forward channel which is received by all mobile terminals of the system. Other, combined frequency/time-division multiplexed channels carry return data traffic from the mobile terminals to the hub.
Both the forward data traffic and return data traffic consist of two traffic components of data. A first traffic component of data necessary for "call-setup" and "call-teardown" includes data referred to broadly as system data and more specifically as network management data. The first traffic component of data is called "signalling" or signalling packets (of data). The term "signalling" is used herein throughout to refer to this first traffic component of data and to a specific transmission mode in communication from the mobile terminals to the hub. A second traffic component of data bears user information, such as messages or data reports. Messages are typically user information, having been composed by the user, while data reports are typically telemetry-type information packets that are transmitted periodically by the mobile terminals. The transmissions of the second traffic component of data to transfer user messages is also referred to herein as "messaging". User information is in the forward direction communicated over typically fixed links to the hub. The hub stores the user information and selectively formats it as the second traffic component of data into the frames for transmission over the TDM forward channel. Systems, such as the Standard-C system are therefore also referred to as "store-and-forward" communication systems.
The Standard-C system communicates at a fixed data rate of 600 bits per second (bps). In communication over the TDM, or forward channel, data of both components are formatted into frames and are transmitted as a sequence of consecutive frames over the TDM channel. The frame length is established at 8.64 seconds, such that during a 24-hour period an integer number of 10,000 frames are transmitted over the TDM channel. The information, message and signalling packets, are scrambled, 1/2-rate convolutionally encoded, and interleaved on a frame by frame basis. Decoding of received frames of information is also done on a frame by frame basis. Considering that the transfer of messages or "messaging" also requires "signalling" in both the forward and the return direction, decoding delays become additive and result in typical message transport delays of several minutes, such that a command-response type of transaction cycle may take place over a time period of about five minutes.
Past applications of the Standard-C system in global communications have traditionally involved communications between a fixed shore station (the hub) and any one of a number of mobile terminals which were ship-based. In such maritime environment, message transport delays of several minutes were not considered to be unacceptable.
In contrast to a relative indifference to time delays in shore to ship communications, user messages between a central trucking dispatch depot and a fleet of operating trucks tend to be more time-sensitive. For example, interactive communication is required when a driver is in difficulty, e.g. by being lost or in misunderstanding with a customer. Urgent messages to alert drivers of additions or deletions in pick-up or delivery schedules while the trucks are already enroute are more the rule than the exception. Thus, if a Standard-C communication system operates between a "land earth station" ("LES") as a hub and a number of mobile terminal equipped, land-based vehicles, existing message transport delays of several minutes for typical command-response type transactions become undesirable.
Increasing the data transmission rate over the standard 600 bps data rate of the Standard-C system would result in a higher message data capacity per frame, thus allowing more mobile terminals to be serviced over the system. However a data rate increase over that of the prior art 600 bps would not alleviate the system's inherent message transport delays. On the other hand, relatively shorter frame lengths could speed up handshake operations between transmitting and receiving terminals and would therefore reduce message transport delays. However, the use of frame lengths shorter than the standard 8.64 second frame length would, at any given transmission rate, tend to increase the ratio of network management data to user message data. Thus, shortening the frame length by simply increasing the data rate proportionally to the frame length reduction does not improve the aforementioned overhead ratio of network management information to user information. The frame structure comprising both network management data and user message data would become compressed in time but remain proportionally the same.
Besides the undesirably long message transport delays which are experienced in the described prior art communication systems, a problem of channel usage is experienced in such prior art systems. Communication systems, such as the Standard-C system, dedicates frequency-division multiplexed return channels as either signalling channels or message channel. Thus, when the mobile terminals communicate with the hub, the first traffic component over the return channels, namely signalling, is assigned to the dedicated signalling return channels. Conversely, the second traffic component of user messages is assigned to the dedicated messaging return channels. It has become apparent that such use dedication of available channels for communication from the mobile terminals to the hub results in an uneven utilization of the available channel capacities. As described above, signalling as used herein refers to the transport of protocol data packets for the purpose of network management, e.g. for call establishment and tear-down. Time division multiplexing within a signalling channel is accomplished by means of the slotted Aloha protocol, a contention-based channel access algorithm that does not guarantee packet delivery. In addition to the transport of network management protocol packets, it is also common to use the signalling channel for the transport of short data reports, provided they can tolerate non-guaranteed delivery, the use of signalling channel for such purpose being referred to as "datagram" service. In contrast, user messages requiring guaranteed delivery are sent on separate frequency multiplexed time-division multiple-access TDMA) channels which are contention free. It is these latter type of TDMA channels that are referred to as "message channels". Access to the message channels is controlled by the hub, and is orchestrated through instructions contained in protocol packets which are sent on the forward TDM channel and acknowledged or responded to over the return signalling channel. When typical user message communication takes place over the Standard-C system, it is common to reach a traffic congestion limit in the TDM forward channel and the signalling channels before reaching a congestion limit in the message channels. Therefore, the message channels often remain still underutilized when the signalling capacity of a particular channel group has become exhausted and further traffic has to be routed to a new channel group.